In wireless communication systems, the quality of the signal depends in large part on the amount of noise measured at the receiver antenna. In these systems, the noise figure is the ratio of the output noise power to the thermal noise in the input termination at standard noise temperature. The noise figure thus represents the ratio of actual output noise to that which would remain if the device itself was noise free, and provides an indication of the performance of a radio receiver. The noise power is typically used to denote the cumulative effects of noise figure at the receiver and the ambient (e.g. non-system) interference. The knowledge of noise power at the receiver is crucial for several blocks in the transceiver chain, which include, but are not limited to demodulation, decoding, power control, link adaptation, and similar operations.
In general, there are three main sources of noise at the receiver antenna: (1) ambient (non-system) noise or interference, which is at or near the same operating frequency range of the desired signals; (2) circuit noise, which is noise introduced or picked up by the circuits or blocks in the RF (radio frequency) stage of the receiver itself; and (3) system or system-like interference, which is introduced by other transmitters or sources of desired signals for other receivers, but not for a particular receiver. In order to design and build effective wireless receivers, it is important to know or at least be able to accurately estimate or measure the signal-to-interference-plus noise ratio (SINR) in order to ensure communication quality or rate of transmission in a wireless link. The SINR level may differ depending on the location of a receiver within a cell, sector or other geographical characteristic of the wireless system, and can also vary depending upon the composition or amount of noise versus interference, or vice-versa.
For wireless transmission systems that utilize OFDM (Orthogonal Frequency Domain Modulation) schemes or similar cellular systems, users who are on or near the boundaries between cells or sectors usually have low SINR ratios because of strong interference from terminals neighboring cells or the large distance from the basestation (transmitter). A multi-user version of OFDM is OFDMA (Orthogonal Frequency Division Multiple Access), which assigns subsets of subcarriers to individual users, thus allowing simultaneous low data rate transmission from several users. OFDMA systems may employ a “frequency reuse-one” technique, in which every cell and sector is free to utilize all of the subcarriers and symbols used in other cells and sectors. Such a system can have significant interference between sectors and cells, especially at the boundaries. In systems with lower frequency reuse, the interference may be reduced at the sector and cell boundaries, but can appear elsewhere. One issue with regard to OFDMA systems with reuse-one (or “reuse-1”) mechanisms is the task of accurately measuring the noise and interference levels. In reuse-one systems, all of the subcarriers are filled with data and pilot traffic from the subscribers. In such a system, or other similar systems, it is difficult to isolate the noise and interference in specific subcarriers. One present method involves cancelling a receiver's signal to measure the remaining noise and interference. Such a cancellation process, however, is not optimum for certain systems or receivers, as some of the original signal may be injected into the measurements. It is desirable, therefore, to provide a system of measuring noise and interference that does not include or implicate the receiver's own signal.